1. Field of Invention
This invention relates to a flame-retardant adhesive composition. More particularly it relates to an adhesive composition for dielectric films and is useful in the fabrication of solderable flexible circuits.
2. Description of the Related Art
The flexible circuit industry requires adhesives for dielectric film and metal foils (usually copper). The laminate, and hence the adhesive, must be able to withstand elevated temperatures and a variety of harsh solvents and chemicals. During the many preparation and processing steps for circuit manufacture, these solvents and chemicals can cause an adhesive to swell, leading to blister formation and/or delamination, which results in reduced circuit yields. The swelling can also open the adhesive matrix to ionics which can result in metal dendrite formation, reduced electrical resistance, and localized darkening or staining. The application of heat, such as in soldering, can similarly cause circuit failures and/or staining. In addition, it has become increasingly important for the adhesive compositions to be flame retardant. External fire sources or short circuits can cause a flexible circuit to burn if it is not flame retardant.
It is known that flame retardancy can be achieved by the addition of materials such as halogen-containing compounds, antimony oxide, or phosphorous compounds. In particular, brominated compounds have been used to achieve flame retardancy in hot melt adhesives, thermoplastic materials, and reinforcing resins, as described in, e.g., U.S. Pat. Nos. 4,727,107, 5,041,484, 5,376,718 and 5,443,911. However, to be useful as a flame retardant in an adhesive for electronic applications, a flame retardant material must satisfy several property requirements which are not necessary for hot melt adhesives or thermoplastic materials. For example, the flame retardant additive must not adversely affect the electrical properties of the composite structure. The additive must also not degrade the adhesive performance of the composite structure, both before and after soldering steps, i.e., the peel strength of the composite structure must not be substantially lower. And, the additive must be stable to processing solvents and chemicals.
Acrylic adhesive compositions with flame retardant additives or fillers for use in flexible circuits have been described in U.S. Pat. No. 5,331,040 and European Patent 429027. However, the addition of the flame retardant filler tends to make the adhesive more brittle, which adversely affects the peel strength of composite structures made with the filled adhesive. The brittleness generally can be overcome by the addition of plasticizers. However, the plasticizers must be chosen such that they are not attacked by processing chemicals, are stable at soldering temperature, etc. In addition, the fillers are generally composed of particles of a micron or more in size and tend to make the adhesive films opaque. Opaque films make inspection processes difficult.
Thus, there is a need for a relatively clear, flame-retardant adhesive composition for use with dielectric films in composite structures for electronic applications, which provides good adhesion throughout the life of the composite structure, and retains all the necessary properties, including electrical properties, of non-flame-retardant adhesives.